1. Field of the Invention
The invention relates to optical transmitters, and particularly to manufacturing an optical transmitter to minimize a temperature-dependent distortion characteristic of an optical signal transmitted through an optical fiber.
2. Description of the Related Art
A modern alternative to sending analog or binary digital information via electric voltage signals over a conductor is to use optical (light) signals over a fiber optic cable. Electrical signals from analog radio frequency or digital circuits (high/low voltages) may be converted into amplitude or frequency modulated optical signals with LEDs or solid-state lasers such as VCSELs or edge-emitting lasers. Likewise, optical signals can be translated back into electrical form through the use of photodiodes or phototransistors for introduction into the inputs of amplifiers, demodulators or other types of circuits.
Laser resonators have two distinct types of modes: transverse and longitudinal. Transverse modes manifest themselves in the cross-sectional intensity profile of the beam. Longitudinal modes correspond to different resonances along the length of the laser cavity which occur at different wavelengths within the gain bandwidth of the laser. Mode hopping occurs when relative intensities at different lines corresponding to different longitudinal modes shift under certain circumstances. In order to provide a reliable communications link utilizing an optical transmitter, it is desired to prevent mode hopping in lasers used in such optical communications applications.
A factor in whether mode hopping will tend to occur in a laser is the degree of stability of the laser. There are many forms of stability, including wavelength stability, pulse-to-pulse energy stability, repetition rate stability, thermal stability, bandwidth stability, among others, and these may be attempted to be controlled in various ways. For example, energy stability and repetition rate stability often depend on the stability of the electrical or optical energy input to the gain medium. The degree of wavelength or bandwidth stability may depend on quality of resonator materials and other factors. The degree of thermal stability may influence the wavelength or bandwidth stability, and may typically depend on the heat capacity of the gain medium and whether cooling and/or heating elements are provided along with a thermal sensor, i.e., a temperature controller, heat exchanger or other such thermal monitor and heat transport device, and what degree of sensitivity of thermal control these devices exhibit. Various developments have been made for stabilizing various parameters of laser systems including operating temperature, and preventing occurrences of mode hopping.
Transmitting digital information in optical form from the optical end may be done in open air, simply by aiming a laser or an array of lasers of a transmitter or transceiver at a photodetector at a remote distance, but interference with the beam, beam divergence, scattering, dispersion, etc., make it difficult to transmit the beam without very significant distortion. One way to avoid the problems of open-air optical data transmission is to send the light pulses down an optical fiber. Optical fibers will transmit a beam of light much as a copper wire will conduct electrons, with the advantage of completely avoiding all the associated problems of inductance, capacitance, and external interference plaguing electrical signals.
Even for single modes of an optical medium, a light pulse emitted by a LED, VCSEL, edge-emitting laser, etc., taking a shorter path through the medium will arrive at the detector sooner than light pulses taking longer paths. The materials that form optical media will impart some degree of chromatic dispersion, or variation of the group velocities of different frequencies of the detected signal. The result is distortion of the amplitude and phase of an information-carrying signal. This problem becomes worse for broader bandwidths or modulation frequencies and as the overall media length is increased. For very long distances, such as in optical communications equipment, even light pulses having very narrow bandwidths will result in signals exhibiting undesirable degrees of distortion.
Optical signals transmitted from a modulated laser transmitter over an optical medium are known to exhibit some degree of distortion, even with single mode lasers very narrow bandwidth and even when steps are carefully taken to stabilize the optical pulses by conventional methods, as briefly discussed above. Electronic pre-distortion circuitry has been developed in an effort to reduce distortion in optical signals, e.g., see U.S. Pat. Nos. 6,288,814, 5,798,854, 5,252,930, 5,132,639, and 4,992,754, which are hereby incorporated by reference. Distortion appears at the output of an external cavity laser (ECL), and is an effect of the laser design. Although there is also distortion presented when the laser is linked to an optical fiber, it is not as great and does not vary as much as the ECL distortion.
Due to significant thermal influences on distortion, pre-distortion circuitries may be preferably configured to operate in dependence upon the operating temperature of a non-linear laser. Multiple pre-distorter settings may be provided such that particular settings, e.g., voltages for controlling pre-distorter outputs, may be selected depending on monitored temperatures of the laser. Moreover, laser output may be monitored through a fiber tap to detect any variation in amplitude or phase, and in response thereto, the laser temperature may itself be controlled and somewhat stabilized, e.g., using temperature sensors and thermal electric coolers (TECs), such that influences of thermal variation on distortion by adjusting the pre-distortion circuitry. Due to its substantial influence on the quality of optical communications signals, it is desired to further control and reduce distortion, particularly in optical signals generated by external cavity lasers (which include a laser chip together with an external reflector).
It is known from the work of other researchers that the degree of distortion of a transmitted optical signal in an external cavity laser is dependent on the temperature of the laser transmitter. Moreover, it is also known from such work that the distortion typically has a discernible minimum at a certain temperature for a single operating mode of the laser transmitter that varies from laser to laser. It is therefore a goal of the present invention to provide a method for manufacturing, and a configuration for a laser transmitter system for optical communications that is thermally-stabilized around that distortion minimum of the utilized laser.